A problem with prior art robotic systems is the poor accuracy and repeatability of the robots, and the resulting inability of robots to put a peg into a close fitting hole. Most automated processes utilize some sort of robotic manipulator for material handling or assembly operations. To increase productivity and quality, critical metrology standards are maintained, but this requires the use of more accurate machine tools, parts, holding fixtures, and finished parts. As a result, method and apparatus to overcome the problems of accuracy and repeatability in robots are needed.
In particular, large robotic manipulators (in the order 36 inches reach and 20 pounds payload) can only be used to repeat previously taught positions at best to within 0.010 inch. However, to make use of such repeatability, it would be necessary to add different robot programs for different cycles of use and environmental conditions. Thus, it is appreciated that there is a need to assist a robot in loading of a part into a collet (peg in hole) or otherwise increase the accuracy of robot movements.
In U.S. Pat. No. 4,362,977 (Evans et al), a manually manipulated teaching robot is disclosed. The teaching robot is used to program the movements of a larger robot. While this increases the ease with which the larger robot may be taught new positions, this does nothing to increase the accuracy or repeatability of the robot motions. In U.S. Pat. No. 4,419,041 (Rose), a system of gears and racks is used to record the three Eulerian angles that a single arm can trace out in space (including twist of the arm about its length) and the extension of the arm via a telescoping tube. Accuracy on the order of 1 part in 7,000 is claimed, but this is an order of magnitude less than required for many large robotic applications. This method is also not applicable to multi-arm structures and does not address environmentally induced errors (heat expansion of metal)
In U.S. Pat. No 4,119,212 (Flemming), a "knee" joint with a planar goniometer attached is disclosed. This system is not structurally stable because a large static error in the measuring system occurs when the linkage is straightened out and gravity applied normal to its length and the axis of the joint rotation. Since the two lengths are connected by an angular measuring device (which is only supported by the links) and supported at their ends by angular measuring devices, no bending moments are transferred about the joint axis. Since no length adjustment is allowed for, the links sag until steady equilibrium is reached. In addition, no out of plane bending moment is measured with this device. This, this device does not provide a means to allow a robot to load a collet accurately.
In "Enhancement of Robot Accuracy Using a Macro/micro Manipulator System" by Andre Sharon, a masters thesis submitted to the Massachusetts Institute of Techology in September 1983, and in U.S. Pat. No. 4,595,334 (Sharon) a micromanipulator is disclosed which is attached to a larger robot manipulator. According to the disclosed system, a large robot is used to carry a micromanipulator to an area of interest. Thereafter, using the micromanipulator for fine motion, an exact positioning of an object is achieved. The micromanipulator has five degrees of freedom including three linear degrees and two rotational degrees. Suitable linear actuators are provided for controlling the degrees of movement. However, the device disclosed in this reference is subject to large friction forces which retard movements along the five degrees of freedom and which retard movements of very limited size in all five degrees of freedom. Also the design is heavy and bulky.
From the above prior art devices, it can be appreciated that reliance on the accuracy of mechanical components is not sufficient. In fact, the basic physics of the situation is that most structural components cannot be machined to 0.0001 inch, which when multiplied by an arm length of 50 inches in three joints, leaves an error on the order 0.0015 inch. Thus, in order for a robot to be able to consistently put a peg in a hole, it must have some sort of compliance mechanism (which is how a human loads a part by feel) or an active measurement system that will allow the robot to servo in on the correct location.
Passive compliance systems (such as the remote center compliance system developed at Draper Laboratory in Cambridge, Mass.) can be useful for inserting parts in a vertical hole. However, the compliance device must be matched with a part weight and only works well in the vertical mode. For a robot loading a collet on a machine tool with a horizontal spindle, a wide variety of part sizes must be accommodated and compliance mechanisms have difficulties operating under these conditions.
Another problem with compliance devices is that they require the robot controller to be able to command the robot to make small dither type motions to get the part into the hole. Use of compliance devices thus requires force feedback so that the robot will not push too hard and damage the system. The majority of robots, however, can only repeat previously taught moves and position servo between previously taught points. This precludes use with respect to search and find algorithms and use of auxiliary feedback from foreign sensors. Thus, in order to use existing robots, these robots would need to be retrofitted with new electronic controllers and force sensor systems whose cost would be relatively large. In addition, commercial software is not available for use for this type of system at present.